A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Before exposing the substrate, it must be correctly aligned to ensure accurate projection of the functional features. Alignment marks are provided on the substrate and detected with an alignment system. Examples of alignment systems are a conventional through the lens alignment system and also the alignment methods and apparatus described in co-pending applications EP02251440 and EP02250235. The alignment marks are commonly on the front side of the substrate, but can also be on the back side of the substrate. Marks on the back side of the substrate are used particularly in the manufacture of Micro Electro Mechanical Systems (MEMS) or Micro Opto-Electro Mechanical Systems (MOEMS). If marks on the back side of a substrate are used, front-to-backside alignment (FTBA) optics project the mark on the back side of the substrate to the front side. Alternatively, if the marks on the front side of the substrate are no longer usable due to, for example, chemical mechanical polishing (CMP), epitaxial layer deposition or thick grainy metal layer deposition, back-to-backside alignment (BTBA) optics can be used. However, for true analysis of front side mark vector behaviour it is required that a set of front side global alignment marks are etched back to remove the process films covering them, in order for all other marks to be measured against the location of these front side global alignment marks. This process is typically used to evaluate CMP processes, and the resilience/accuracy delivered by segmented CMP marks. The problem with this art is that it is a slow and costly process that cannot be conducted in real time as a quality control gate. It requires short loop experiments to be run rather than using production substrates. Moreover, the polishing process for CMP is very complex and changes from substrate to substrate, so it is virtually impossible to accurately measure, and adjust for, every substrate's asymmetric/non-linear behaviour.